Traditional incandescent lighting devices have a natural omnidirectional light spreading since the upper portion of the filament is spaced from the screw base of the lighting device. Consequently, light is emitted not just forwardly and laterally, but also backwardly. Conventional solid state based lighting devices, such as light emitting diode (LED) based lighting devices, has a more directed light spreading compared to such incandescent lighting devices since the light source (such as the LED) is itself flat and is normally attached to the base of the lighting device for providing sufficient heat dissipation from the light source via the heat sink arranged at the base. Therefore, the light spreading pattern of LEDs is normally of the Lambertian type, which means that light is emitted mainly forwardly from the lighting device.
In order to resemble the more omnidirectional light spreading of a traditional incandescent lighting device, solid state based lighting devices may comprise a light guide for guiding light emitted by the light source and emit it from a position remote from (such as above) the base of the lighting device. Another alternative is to provide an optical feature for creating a virtual light source above the real light source, e.g. by redirecting and/or refracting light from the light source. Thus, the virtual light source is spaced from the base of the lighting device and therefore spreads light in lateral and backward directions. An example of such a lighting device is shown in US 20120320580 A1. A drawback with such a lighting device is that the optical cover (or envelope) comprising the optical feature for refracting light may be complicated and expensive to manufacture.